test(theme-studio): cover defensive branches and the palette generator

Added the uncovered fallback branches in app-core (migrateLegacyFace null input, normalizePkgFace's source fallback chain, mergePackagesInto's null/new-app guards, boxCss shading a relief from the bg when no box color is set) and in colormath (apca's equal-luminance return-0 and low-contrast clamp, isPureEndpointHex). New test-palette-generator-core.mjs drives planPaletteGenerator across every scheme, vibe, source mode, and the fill-gaps intents, since those internals are only reachable through the public planner. colormath branch 96 -> 99%, palette-generator-core funcs 97 -> 100%, node suite 237 tests. The remaining gaps are the deep palette-column edge branches, deferred as diminishing returns on already line-covered code.

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